How does this relate to Classic? Classic won't be going anywhere.
There will come a time when you will be sitting there in your capital, in your shiny gear, bouncing or sitting around, with nothing to do. Yes, even in vanilla you can run out of content.
...
Without additional content, the majority of the people will move on, your guild will die, and you'll be sitting there in front of your PC, a single tear running down the side of your cheek, wishing Vanilla would go on.
But they won't "run out of content" because people are running off to "fresh servers" (at least not fresh Blizzard servers), they're leaving because they're simply sick of the game.
See, the beauty of a static server is that -- it's static! There's no progression! If you get there day one or day 1000, it's the same server (save for having a more mature economy).
So, you're not missing anything. If you like leveling Dwarves and running through instances, and what not -- you can do that. If other folks like doing that, you can group with them.
If there's no one else in the game, you can still level your characters. Harder to do group content, but, c'est la vie. Organize a group on the forums. Folks will organically migrate to the servers with the people.
"Hi I just rolled on Fizzbang server, and there's no one here!" "Yea, Fizzbang has been dead for a year, you should re-roll on Gogglesprocket, we have a great community!"
The problem with progression and the "fresh server" concept is simply that, there's always this drive to race through the current server and then jump to another. Or, worse, "well, the new server lit up, and everyone left, so I may as well quit."
Without progression, you don't have events like that. People will leave when they're done, not because of something shiny offering them the same thing.
Progression kills Classic, since people will always be looking for what's new, what's next, "I'll just wait for the next patch." But static Classic is static. No impetus to leave or wait. It just is, an open book to browse. Not a Part 1. You want to see the rest? WoW is over there.
But see, there will be new 13 and 14 year old kids every day that have NEVER played on "Classic". They may like WoW, they may be sick of WoW, but then they'll try Classic -- and off they go! Because Classic is still there in all its Classic glory.
Did you know the people still buy and play "Monopoly"?
WoW is the best advertisement for Classic. Not because one is better than the other, just simply that it exists. Every time a player launches WoW, there's going to be a Classic button staring at them. In a few mouse clicks, they be in Classic doing Classic things.
Classic will be a playable museum. Cheap enough for Blizzard to keep the lights on indefinitely. People will ebb and flow, and the communities will self organize organically.
Classic won't need anything.